THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.

WOLF BLITZER, CNN HOST: Happening now, Trump's troubles. He's heading into the pivotal Wisconsin primary, trailing in the polls and off-kilter from a week of controversies. Is his presidential campaign taking a turn for the worse?

Ruining the party. GOP officials are bracing for an ugly contested convention. Did their closed-door meeting with Donald Trump influence their plans and give them any hope for unity? I'll ask the RNC spokesman, Sean Spicer.

Sick of it. Hillary Clinton accuses Bernie Sanders' campaign of lying, telling an activist that confronted her that she's downright fed up. Will the next big contest in Wisconsin add to her frustration?

And nuclear terror. Amid growing fears ISIS may get a dirty bomb, President Obama is warning the world to be on guard. He has a news conference this hour. We'll carry it live.

We want to welcome our viewers in the United States and around the world. I'm Wolf Blitzer. You're in THE SITUATION ROOM.

Tonight, Donald Trump is wrapping up what's arguably been the worst week of his presidential campaign, and it's not clear if next week will be any better as he heads into Tuesday's Wisconsin primary trailing Ted Cruz by ten points in a new poll.

Trump's opponents are ratcheting up their claims that he's unprepared to be commander in chief, citing his controversial remarks on abortion, his provocative foreign policy views, his verbal attacks and more.

A day after Trump met with Republican National Committee leaders, there are serious doubts about his promise to try to unify the party, as the likelihood of a contested convention looms.

And our correspondents and analysts, they're also standing by as we cover the day's top stories.

Up first, CNN's Sunlen Serfaty, she's joining us now from Wisconsin. Sunlen, voters there will have a say on whether Trump bounces back. What's the latest?

SUNLEN SERFATY, CNN CORRESPONDENT: That's right, Wolf. And Donald Trump is preparing to head to Wisconsin tomorrow ahead of that very key primary for him on Tuesday night here in Wisconsin, where he now faces this big question: what will the effects of this very rough week on him be, and will it lead to any lasting damage?

TRUMP: Wisconsin is very important. Who's going to vote for Trump on April 5?

SERFATY: Tuesday's Wisconsin primary now a big test, as the GOP front-runner tries to rebound from the toughest week of his campaign.

TRUMP: It's a very serious problem.

SERFATY: Trump is attempting to clean up his controversial comments earlier this week, when asked if women should be punished for having an abortion if it became illegal.

TRUMP: The answer is that there has to be some form of punishment.

SERFATY: But he then reversed his position. Now a rare admission from the billionaire.

TRUMP (via phone): It could be that I misspoke, but this was a long, convoluted subject.

SERFATY: Yet, Trump is not backing down from another firestorm he sparked this week, standing by his refusal to rule out using nuclear weapons in Europe.

TRUMP: Europe is a big place. The last person to use nuclear would be Donald Trump. That's the way I feel. I think it is a horrible thing. The thought of it is horrible. But I don't want to take anything off the table.

SERFATY: Trump's rivals are not letting up their attacks.

GOV. JOHN KASICH (R-OH), PRESIDENTIAL CANDIDATE: The problem for him with town halls is he actually has to answer questions in a specific way.

SERFATY: That backlash comes as Trump aims to mend fences with the Republican Party, sitting down Thursday with officials at the RNC, including Chairman Reince Priebus.

REINCE PRIEBUS, RNC CHAIRMAN: We talked about unity and working together and making sure that when we go to Cleveland and we come out of Cleveland, that we're working in the same direction. SERFATY: A source familiar with the meeting tells CNN that Priebus

told Trump that his disparaging comments about the RNC made it difficult with donors and activists whose help he'll need if he becomes the nominee.

TRUMP: Actually a terrific meeting, I think. And it's really a unity meeting.

SERFATY: Even as he talks about unity, Trump isn't backing down from the campaign fight, releasing a new Instagram video today targeting Ted Cruz.

SEN. LINDSEY GRAHAM (R), SOUTH CAROLINA; I think he's been just as wrong as Obama if not worse.

SERFATY: While Ted Cruz is staying on the offensive against Trump, dubbing his rival a reality TV show candidate.

SEN. TED CRUZ (R-TX), PRESIDENTIAL CANDIDATE (via phone): Donald Trump is the Kim Kardashian presidential candidate. He sits on Twitter and makes a lot of noise, but he has no solutions to fixing the problem.

SUNLEN: That as Cruz becomes more of a target for John Kasich, who's ratcheting up his attacks on the Texas senator.

KASICH: The problem with Senator Cruz is he has no record. His record is shutting down the government and making everybody he works with upset.

(END VIDEOTAPE)

SERFATY: And as Donald Trump continues to insist that he will unite the Republican Party, a poll out shows just how much of an uphill climb that will potentially be for him. Only 38 percent of Republican voters say that they will unite solidly behind Donald Trump -- Wolf.

Joining us now the Republican National Committee's chief strategist and communications director, Sean Spicer.

Sean, thanks very much for joining us. I know there was a major meeting yesterday. Donald Trump met with your boss, Reince Priebus, leaders of the RNC. How did it go?

SEAN SPICER, RNC CHIEF STRATEGIST AND COMMUNICATIONS DIRECTOR: Well, first I'd like to say that the chairman speaks to all the candidates in the campaigns on a fairly regular basis, so I know that this was big news in Washington, but on a weekly, sometimes daily basis, he's speaking to most of these campaigns. So first that.

Second, it was a great meeting. I think that they had an opportunity to talk about the state of the race, delegate math, where the race is going, and just share some thoughts on where things are heading from now. And on planning, frankly.

BLITZER: So he's had similar meetings with Ted Cruz, with John Kasich at RNC headquarters here in Washington like he had yesterday with Donald Trump?

SPICER: Well, I mean he e-mails and calls them. So, no, I mean, if you want to talk about exactly where it was, you know, we've -- but he's met with Senator Cruz, he's met with Governor Kasich. So he's in constant contact. This happens to be they mentioned they were going to be in town, and we asked them to swing by.

So I don't think this is that big of a deal. He's in constant contact with all three of the remaining candidates.

BLITZER: Which is important. Trump was very pleased with the meeting. You heard what he said and he also tweeted this: "Just had a very nice meeting with Reince Priebus and the GOP. Looking forward to bringing the party together, and it will happen." Will it happen?

SPICER: I think so. Absolutely. I mean, every one of the candidates needs to do what they -- what they have to right now. They're focused on winning the 1,237 necessary delegates to become our nominee.

But what unites all of them, frankly, is the fact that when we come out of Cleveland, we need to come out as a united party and that one thing that everyone agrees, no matter where you are in this party, is that Hillary Clinton is not an option that's acceptable. So we will get together in Cleveland. We will come out unified as a party, and we will go on and win in November. But I'm absolutely confident of that.

BLITZER: Did they discuss the possibility at that meeting you had yesterday of a contested convention developing in Cleveland in July?

SPICER: I didn't -- I don't think it necessarily came up that way. There was just an overall of where the delegates were now, what the math looked like going forward. So I'm not going to get into specifics, but it was much more of a general state of the race, where things are headed.

BLITZER: I know the three remaining Republican presidential candidates in their own respective ways, they've all walked away from that so-called loyalty pledge. I know the RNC wanted them to sign that pledge, promising they would support whoever emerged as the Republican nominee, not run as a third-party candidate. They've all walked away from that. How disappointed are you about that?

SPICER: Well, look, I think all of them do recognize that the role the party plays is extremely important to our victory in November. I think all of them understand that unity is important. So I know that they're saying what they have to say at this point.

We're in a primary. It's definitely competitive. Everyone understands what's at stake. We're in a very, very key part of this cycle. And I think they're saying whatever they have to say to make sure that their supporters, you know, stick with them, that they build support.

But look, Wolf, I know the discussions on our side but as everyone watching and everyone in D.C. knows, the big rumor that's been going around in D.C. is when Hillary Clinton, if she gets indicted, there's a plan afoot to try to put Vice President Biden in and how that's going to play.

I think the Democrats, what's looming on their side, if this indictment plays out, which a lot of people are speculating it is, and this sort of underground movement to figure out how to put the vice president in the place is going to cause equal -- I mean, catastrophic shifts in what's going to happen in the Democratic Party. You're going to have Sanders folks going after it, going nuts.

So I think that, like, the story today may be what's happening in Cleveland, but I'm confident that we're going to put on a fantastic convention and come out unified. But I think the looming story is what's going to happen on the other side.

BLITZER: Well, when you say when she gets indicted, how do you know she's going to be indicted?

SPICER: No, I'm sorry, I said "if." I think that's -- there's a lot of -- what you're hearing a lot from Democrats right now is what's the plan if she does?

BLITZER: If.

SPICER: I think that there's a lot of concern based on the evidence that's been presented in public that that's a high possibility. So there's no question that when you talk to people that are involved in understanding the nuances of this, there is a lot of concern on their side of the aisle about what is that plan B going to be, because they realize that, while Bernie Sanders has done a phenomenal -- run a phenomenal campaign, and this is not the race that she expected, they've got to figure out how they're going to ensure that he's not their ultimate nominee.

BLITZER: We know that the FBI by all accounts, they seem to be getting closer and closer to wrapping up that investigation into her e-mail server. Let's see what happens over the next several weeks as they wrap up this investigation.

[17:10:09] Sean, as you know, there's a new poll out, put out by the Pew Research Center, that found that, what, just 38 percent of the party would, quote, unite solidly behind Donald Trump if he were the nominee; 56 percent say disagreements with the party would keep many Republicans from supporting him. That's much lower than the share of Republican voters who expected the party to unite behind Mitt Romney, 65 percent, John McCain, 64 percent, at comparable points in the 2012, 2008 presidential campaigns. Does that worry you?

SPICER: No, I think it's a very different cycle. I mean, I don't think both -- Mitt Romney was a presumptive nominee in early April. John McCain was the presumptive nominee in March of their respective cycles. I think this time right now, you've got a lot of folks that are still very passionate about the remaining candidates.

So it's not that odd that -- that odd that they wouldn't obviously still align and want their individual to win and say, "I'm only going to be in it for my guy."

But look, history has proven on both sides of the aisle that once you have a nominee, the party unites around them. So again, I know that that's the big talk right now but I feel very confident that our party is going to unite. Everyone understands what's at stake. Hillary Clinton herself the other day made it very clear that the next president could decide several Supreme Court justices.

I think everyone who's a Republican and a conservative understands the tremendous impact that that would have on the next generation of Americans.

BLITZER: All right. Sean Spicer, I'm going to have you stand by. We have more to discuss. John King is also standing by. He's going to have a closer look at the delegate count when we come back. Much more with Sean Spicer right after this.

Sean, stand by for a moment. I want to take a closer look at where the Republican delegate numbers stand right now. It's critical. Our chief national correspondent, John King, is over at the Magic Wall for us. John, take us through the fight for delegates in the immediate days ahead.

JOHN KING, CNN CHIEF NATIONAL CORRESPONDENT: Immediate, Wolf, days ahead means Wisconsin, and then we springboard there from New York, Donald Trump's home state. Let's look at the state of play where we are: 739 for Trump, 460 for Ted Cruz in second place. He's significantly back. So what happens?

Well, if Donald Trump can somehow win Wisconsin, which is up on Tuesday, take most or all of its delegates, boy, that would take the steam and the air out of the "stop Trump" movement, right, because they've spent so much time, so much money against him in that state.

But two polls in a row that show Ted Cruz with a ten-point lead. So let's assume that Cruz not only wins statewide on Tuesday, but that he wins big enough to take most or all of the Wisconsin delegates. Some are awarded statewide, the others by congressional district. If you're winning by ten points statewide, just like Donald Trump did in South Carolina, you've got a pretty good chance of taking most or all.

So let's say under that scenario, Wolf, Ted Cruz gets a big win in Wisconsin.

Then we move to Donald Trump's home turf of New York. And again, the calculations become critical. One poll out shows Donald Trump above 50 percent. If Donald Trump loses Wisconsin but then can bounce back in New York and be above 50 percent, then he wins them all, 95 delegates in the state of New York. If that happens, sure, Trump does not want to lose Wisconsin, but if he can rebound with a winner-take- all scenario in New York, what does that mean? It means he's well ahead. And from this point on Donald Trump, if he wins them all in New York, even if he gets shut out of Wisconsin, at this point he would need to win about 53 percent of the remaining delegates. That's not easy, but 53 percent to get to the magic number of 1,237 is doable.

But here's the other scenario. What if Ted Cruz does win Wisconsin and gets most or all of the delegates, and then we get to New York and Donald Trump falls below 50 percent? Even if he wins, this has him winning at 40 something percent. Even if he comes in first -- let's say John Kasich came in second, Ted Cruz came in third. You see the difference isn't all that much there. But if that happens and Donald Trump gets only 40 something as opposed to all 95, at that point, Wolf, the "stop Trump" forces will think that we are almost guaranteed an open or contested convention.

Why? Because if Donald Trump doesn't get them all there, if he gets shut out in Wisconsin or only gets a couple and then has to split the delegates from New York, at this point here, Donald Trump would need 60 percent if that's what happens in New York. If it's not a winner- take-all for Donald Trump, but he has to split the delegates, then he needs six in ten of the remaining delegates. Doable, maybe, but much harder.

So what happens in Wisconsin and then whether or not New York goes winner-take-all or gets split proportionally will dramatically change the math about the prospect of an open convention. And if Donald Trump has to split those New York delegates, you can almost start to bet on an open convention.

BLITZER: Wisconsin this coming Tuesday, New York state two weeks later, critically important.

John, don't go too far away. I want to bring back Sean Spicer of the Republican National Committee.

Sean, you saw that "Washington Post"/ABC News poll. It paints a pretty bad picture for the Republican Party. Trump right now, assuming he's the nominee, right now he has a 67 percent overall unfavorable rating.

But take a look at the unfavorable rating he has among some key groups that Republican strategists had hoped to improve on since the loss in 2012. White women, 68 percent unfavorable. White college grads, 74 percent unfavorable. Young people, 18 to 34, 80 percent unfavorable. Hispanics, 85 percent unfavorable. African-Americans, 80 percent unfavorable. "The Washington Post," in reporting these numbers, they concluded with this ominous sentence: "That GOP autopsy into what went wrong in 2012 has been torn to shreds and scattered to the winds from the top of Trump Tower."

Is it as bad as all of that? [17:20:00] SPICER: Well, first of all, elections are won state by

state in the Electoral College. When you look at that, I think we're doing very well; we're very competitive. Again, and I think that, you know, looking at it myopically, it tells a really interesting story. But when you factor in the fact that Hillary Clinton, their likely nominee, if she's able to become that, I think is no -- no -- something to write home about. Her negatives are unbelievable. Her own base has a problem with her.

If you look at where it is right now, Bernie Sanders is winning young women from Hillary Clinton, who wants to be the champion of women. There's a lot of groups that Hillary Clinton has major, major problems on.

I think we at the RNC have spent our time reaching out, working with minority groups, building out coalitions in key states, in key battlegrounds so that when this election turns from a primary to a general, we're going to be well-equipped to hand our nominee the resources they need to grow their majority into a winning strategy for November.

SPICER: Well, obviously I'd love better numbers for all of our guys, but I think, again, they're focused on their primary. They're focused on trying to win. They're in a very competitive race right now. But I'm confident that the work that we've done over the last four years, Chairman Priebus, as you know, in that Growth and Opportunity Project, has made reaching out to groups that we haven't done well in the past, a top priority both in terms of our data with those groups and our outreach. 2014 showed that we did very, very well and that we were well on the way.

I think when we partner with that nominee and provide the resources that the chairman has built up out through building it out in the states over the last four years, we're going to have what it takes to build a winning coalition. I'm confident of it.

BLITZER: Trump had a bad week, I think it's fair to say, by all accounts. As you know, his campaign manager, Corey Lewandowski, was charged with simple battery. Trump had some awkward comments about punishing women who have abortions. He later revised his position, changed it. Said he's interested in Japan and South Korea getting nuclear weapons, perhaps, to deal with the North Korean nuclear threat. How does this affect the overall Republican Party and its ideals?

SPICER: Well, I think at this phase, when we're in a primary, each of these candidates is trying to do what they need to do in representing themselves. As we go into a general election, that's when we at the RNC will continue to work with them to talk about issues and tone and things that are going to help us win in November. But at this point it's up to each of these candidates to do what they have to do to win -- win the primary. BLITZER: And then we'll see what happens out of Cleveland at that

Republican convention. Sean, you're going to have your hands full, I am sure. Thanks very much for joining us.

SPICER: You bet, Wolf. Have a great weekend.

BLITZER: Sean Spicer is the chief strategist, communications director at the RNC.

Coming up, President Obama on the threat of nuclear terrorism. We're standing by for his news conference. It's coming up this hour. We'll have it live for you.

And Hillary Clinton lets her frustration show, as she fails to shake off a strong challenge by Senator Bernie Sanders. The Democrats, they're getting very intense right now. We have details coming up.

(COMMERCIAL BREAK)

[17:27:34] BLITZER: Tonight Donald Trump is uncharacteristically laying low, but his roller-coaster presidential campaign is making some heads spin, some stomachs turn within the Republican Party.

Let's bring in our chief political analyst, Gloria Borger. Our chief national correspondent, John King, is still with us; our CNN Politics executive editor, Mark Preston; and CNN political commentator Michael Smerconish is with us, as well.

First of all, Gloria, your reaction to what you just heard from Sean Spicer, the RNC strategist?

GLORIA BORGER, CNN CHIEF POLITICAL ANALYST: I think he's a really good tap dancer, Wolf. I think -- I think Sean, when you look at the numbers you showed him about the unfavorable ratings of Donald Trump.

BLITZER: Especially those key demographic groups.

BORGER: Key demographics like women and just the general population. I mean, yes, and his point is completely accurate that Hillary Clinton is also very polarizing and very unpopular.

But when you look at the unfavorables of Donald Trump and the -- and the sort of primaries he's heading into Wisconsin could be a real problem for him, you know, I think that Sean Spicer and the RNC and Reince Priebus, they've got a real problem on their hands that they have to try to get under control in some way, shape or form, which is why their meeting with Donald Trump yesterday was so smart and so important for them to have, because he came out with a unity theme.

BLITZER: He certainly did. He promised that there would be unity. But John, could the Republican Party see some significant changes between now and July when the convention takes place in Cleveland?

KING: Look, I think the three most important words in this conversation are "I don't know." We don't know in the sense that this is such a roller coaster. If Trump can get close to 1,237, then that changes the dynamic of the

convention. If he's very, very close, it's hard to take it away. But if he's in the 1,100 range, and they're trying to take it away, then will the Republican Party see changes? The Republican Party will see uncharted territory, where you'll have an open convention, where nobody really knows what the rules will be, because they'll have to write the rules for that convention. They will use the rules from the last convention as a template, but they'll want to change some of them because of the restrictions they place on nominating people.

But then remember, even as we focus on that, there will be 55 or 57 other sets of rules. Every state and territory that sends a delegation has to write its own rules. Are you bound on the first ballot? What does that mean if you try to defy that pledge?

So we have no clue is the answer here, which is what makes it so exciting. If you're in Sean Spicer's chair or Reince Priebus's chair, a little scary.

BORGER: Can I also...

MARK PRESTON, CNN POLITICS EXECUTIVE EDITOR: Not exciting.

BORGER: In any delegation, by the way, what are the limits about what a candidate can do to try and get somebody over to his team? You know,

[17:30:03] Can you fly a plane load of people down to Mar-a-Lago, and say enjoy the weekend? You probably can.

(CROSSTALK)

PRESTON: They're not governed by the federal government, right?

BORGER: Right, exactly.

BLITZER: That convention rules committee meets a week before the convention and they can come up with whatever rules they want.

BORGER: Sure.

BLITZER: Michael, a lot of people suggest this has been the worst week of the Trump candidacy. Could all these distractions and the comments that we know all about this week actually mean some real trouble for his race for the White House? Because in the past over these past several months we've been down this road before. He seems to have a Teflon coating.

MICHAEL SMERCONISH, CNN ANCHOR, "SMERCONISH": I think they come at a horrible time for Donald Trump. And I was paying close attention to John King as he was working the magic wall and drawing emphasis not only to Wisconsin but also to New York. And then I would add to that list Pennsylvania. I think this is the most critical juncture that's about to play itself out.

And what I'm mindful of, Wolf, is that when you get into a state like New York and when you get into a state like Pennsylvania, the party establishment plays a very significant role. New York changed their rules pertaining to delegates just last year. In Pennsylvania, we don't have a direct election of delegates committed to a candidate in most instances. And the point is party regulars, GOP committee people, donors, are going to fill those chairs in the convention hall from those two states.

If Donald Trump can't get there on a first ballot at 1237, I don't think that favors him on ballots two through however many it takes.

BLITZER: Because on the second round, a lot of those delegates as we know who are committed, who are bound by what the states did, they become sort of free agents. Most of them on the second round, almost all of them on the third and fourth round.

Mark, Wisconsin this Tuesday, then New York two weeks later. Pennsylvania, as Michael just pointed, all huge contests. There are three other contests you're looking at as well.

PRESTON: Right. Let's color all the lines over the next couple of next. You know, we're so used to having a plethora of contests every Tuesday. It's going to slow down a little bit but these are very important starting this weekend. North Dakota is going to hold a state convention. On board, 25 delegates. Why that's important? They're all unbound. They're going to be elected. They do not have to choose a candidate.

Let's fast forward over Wisconsin. On April 9th Colorado holds its own Republican convention. It starts tomorrow. They are going to pick 34 delegates in total, 21 this week, 13 on Saturday. What's significant, they're all unbound if they choose to be. They can pledge themselves, otherwise they are unbound.

And then on April 16th, the Republicans in Wyoming will hold their state convention. Another 14 unbound. So two very important words as we head into the New York primary, right, over the next couple of weeks.

Electability, which is what we're hearing right now from Ted Cruz and we're hearing right now from John Kasich in head-to-head matchups against Hillary Clinton in momentum, because it is about momentum now.

BORGER: You know, and these delegates, by the way, and I was talking to a delegate hunter for John Kasich, Charlie Black who --

PRESTON: Is that a new term, delegate hunter?

BORGER: Delegate hunter. Yes.

PRESTON: I like that.

BORGER: We should do a TV show on that.

PRESTON: We could do -- yes.

PRESTON: Delegate hunter, you know, he points out accurately that delegates are different from primary voters. Delegates care about electability.

PRESTON: Right.

BORGER: They're party people. They don't have a message to send to the national party, they are the party. So they're going to care about who they think can win in the fall, so it's a whole different game right now that Donald Trump is playing and Ted Cruz is playing and John Kasich is playing.

BLITZER: John Kasich's only hope right now.

BORGER: Exactly. Well --

BLITZER: Since mathematically he can't get to that magic number of 1237.

BORGER: That's Charlie Black's point.

BLITZER: He's hoping there'll be a contested convention round two or three. They say, you know what, Kasich might be the most electable.

You know, Mark, I just want to go through some global security issues with you because the president is about to speak out on nuclear weapon, the threat from ISIS getting some sort of dirty bomb. Donald Trump, he weighed in dramatically in recent days on the whole issue of the NATO role in fighting terrorism. Listen to what he said.

(BEGIN VIDEO CLIP)

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: Let me tell you, NATO is obsolete. It was 67 years or it's over 60 years old. It is many countries, doesn't cover terrorism, OK. It covers the Soviet Union, which is no longer in existence. And NATO has to either be rejiggered -- you know, changed for the better. I'm not saying -- the other thing that's bad about NATO, we're paying too much. We're spending a tremendous -- billions and billions of dollars on NATO.

(END VIDEO CLIP)

BLITZER: Our CNN reality check team as you know, Mark, they took a closer look into what Donald Trump is saying about NATO being obsolete, not really involved in counterterrorism. What did they find out?

PRESTON: Well, let's look into his statement, Wolf. By way of background, NATO was created in 1949 to address the threat of the Soviet Union. NATO's mission statement states in part that member nations agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against all of them. In the 1990s NATO intervened to help stop genocide in the Balkans. And then following the September 11th terrorist attacks NATO led the military intervention into Afghanistan.

[17:35:01] In October 2011 NATO also launched an operation Active Endeavor, which to this day has NATO naval vessels patrolling the Mediterranean and monitoring shipping to help deter, defend, disrupt and protect against terrorist activity.

As far back as 2004, NATO established a program to defeat terrorism that included sharing technology among member nations to protect troops, civilians and critical infrastructure. So even though it was established 65 years ago in response to the Soviet threat, it does in fact address the current terrorist threat.

BLITZER: All right. Everybody, stand by. We're going to take a quick break. We have much more to assess, much more to discuss. We're standing by to hear from the president as well. We'll be right back.

(COMMERCIAL BREAK)

[17:40:28] BLITZER: We're following some surprising new developments in the Democratic presidential race. A Greenpeace activist provoked a rare public flash of anger by Hillary Clinton who went on to accuse Bernie Sanders' campaign of lying about her.

Let's go to our national correspondent, Jason Carroll.

Jason, what brought about Hillary Clinton's flare-up?

JASON CARROLL, CNN NATIONAL CORRESPONDENT: Well, clearly it's a little bit of back and forth, Wolf, about whether or not she accepted money from the oil industry, from the fuel industry. Sanders stands by that allegation, stands by that claim. Looking forward, he says he's got the momentum, he's got the money. He says he's going to take Wisconsin and then, Wolf, he says he's going to go on to do the same thing in New York.

HILLARY CLINTON (D), PRESIDENTIAL CANDIDATE: I just go crazy when I hear Senator Sanders and the Tea Party Republicans railing against the exports-import bank.

CARROLL: Clinton holds a significant lead in the delegate count but on the heels of a string of western victories, Sanders is vowing to carry on until the July convention.

SANDERS: If we win here in New York, we are going to make it to the White House.

CARROLL: Sanders is challenging Clinton in her adopted home state of New York, rallying more than 18,000 supporters in the Bronx Thursday night. And here in Wisconsin, polls show him with a narrow lead other Clinton ahead of Tuesday's primary. Despite his delegate gap, Sanders insists there is still a path for him to win the nomination.

SANDERS: You've got 60 percent, 70 percent, 80 percent of the vote in a state, you know what, I think superdelegates should vote for us.

CARROLL: Sanders is likely to have the resources to push forward. His campaign saying it raised $44 million in March, eclipsing its $43 million haul from February.

The hard-fought race growing more intense by the day, with Clinton being confronted by a climate activist after a New York event Thursday night.

UNIDENTIFIED FEMALE: Thank you for tackling climate change. Will you act on your word and reject fossil fuel money in the future on your campaign?

CLINTON: I do not have. I have money from people who work from fossil fuel companies. I am so sick -- I am so sick of the Sanders campaign lying about it. Sick of it.

CARROLL: Sanders today rejecting the charge that his campaign is lying about Clinton's donations.

SANDERS: If people receive money from lobbyists of the industry, I think you're receiving money from the industry.

CARROLL: The Democratic rivals also trading jabs over abortion. Clinton is accusing Sanders of not denouncing Donald Trump forcefully enough for his comments that women who have abortion should be punished if the procedure became illegal.

CLINTON: Senator Sanders agreed that Donald Trump's comments were shameful, but then he said they were a distraction from, and I quote, "a serious discussion about the serious issues facing America."

CARROLL: Sanders is charging Clinton with misrepresenting his record.

SANDERS: What Secretary Clinton did is take things out of context. I am 100 percent pro-choice.

(END VIDEOTAPE)

CARROLL: Well, no doubt -- no doubt that Sanders can draw a crowd, whether he's here in Sheboygan, Wisconsin, or whether or not he's in the Bronx. But going forward, he's got to win. He's got to win big here in Wisconsin, must-win big again in New York, following up with that in Pennsylvania.

Again, Wolf, The reality is he can draw a crowd but he still has an uphill battle ahead of him -- Wolf.

Michael Smerconish, you know, you just heard, $44 million he raised last month, 97 percent of that he raised online. He's got money. He can go the distance if he wants.

SMERCONISH: And what normally drives someone out -- just take a look at what happened on the Republican side of the aisle -- they dropped not so much when the numbers had dropped but when the fundraising fell through the floor.

And $27 at a time, Wolf. He just maintains his perseverance in this. I think he does continue all the way into June, perhaps all the way to the convention. One of the things I'm now wondering is what kind of an impact, assuming she wins the nomination, what kind of an impact might he have on her vice presidential pick.

BLITZER: That's a good question. And John King, you and I have been at that magic wall, you've shown us how difficult it would be for Bernie Sanders to beat her in terms of pledged delegates. She has a lot of these superdelegates, maybe 400 more than he has right now. But can he wean them away from her?

JOHN KING, CNN CHIEF NATIONAL CORRESPONDENT: If he keeps winning maybe. Most of those superdelegates think that he's a weaker general election candidate even though the polls at the moment suggest he's as strong or stronger than her as a general election candidate. They don't think that's right. However, the key is, can he keep winning.

[17:45:02] If Bernie Sanders, Wolf, won every single contest left, he won't, but if he won every single contest left, 55-45, guess what, Hillary Clinton would still lead in the delegate chase. Now that would scare the superdelegates. Bernie Sanders wins in Wisconsin, then battleground New York becomes huge in this race. Can she defend her firewall in New York, and for somehow Sanders can win Wisconsin and then stun her in New York, she'll still lead in the delegate game but, boy, will we be having a different conversation.

BLITZER: It would be amazing indeed. We'll watch every step of the way.

Guys, stand by. Two very important programming notes for our viewers. Tune in tomorrow morning, Saturday morning, 9:00 a.m. Eastern for "SMERCONISH." You're going to want to see that show. I watch it every Saturday morning. And Sunday morning 8:00 a.m. Eastern, John king hosts a special hour-long edition of "INSIDE POLITICS" only here on CNN. Two excellent shows Saturday morning and Sunday morning, catch them both.

In other news, there was a chance that the Brussels airport could have been reopened this evening, but it did not. A temporary check-in hall is ready but airport workers, they are threatening to strike unless security is tightened.

It's been a week and a half since suicide bombings at the airport and a subway station killed 32 people, injured 330 more.

CNN's "Impact Your World" team has gathered ways you can support and remember the victims. Go to CNN.com/impact and you will be able to impact your world. Stay with us. We're standing by right now for President Obama's live

news conference. He's wrapping up a nuclear security summit dealing with the threat of ISIS getting some sort of nuclear bomb, some sort of radiological dirty bomb among other things. The president and dozens of world leaders, they're trying to make sure terrorists don't get their hands on nuclear material.

Up next, why one key leader could be a big help, but is not attending.

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[17:51:31] BLITZER: We're awaiting President Obama's news conference from the site of today's Nuclear Safety Summit here in Washington. The leaders of some 50 nations, they've been here in Washington over the past few days. But one key player who could help keep nuclear material out of the hands of terrorists is not taking part in the summit.

Let's go to CNN's Brian Todd. Why is Vladimir Putin not here, Brian?

BRIAN TODD, CNN CORRESPONDENT: Well, Wolf, one reason we all know. Vladimir Putin and Barack Obama simply don't like each other. That's part of the dynamic. But the Russians also feel that they and their allies have been marginalized at this summit. Now the key question tonight, does Putin's absence give ISIS any kind of advantage in getting its hands on nuclear material?

(BEGIN VIDEOTAPE)

TODD (voice-over): With the Brussels and Paris attacks, a clear message from ISIS. They can and will hit Western cities and kill as many civilians as they can. And it appears ISIS wants to create more devastation. After the Paris attacks, investigators in a raid found surveillance footage of an employee at a Belgian nuclear facility.

ANDREW BIENIAWSKI, NUCLEAR THREAT INITIATIVE: And that nuclear facility had highly enriched uranium but also produced these radiological sources. So now we know that they actually are trying to take steps to try and acquire these materials. And so therefore we need specific action coming out of this Nuclear Security Summit.

TODD: But at the Nuclear Security Summit in Washington, one leader who could make a huge difference in securing nuclear material was a no-show. Vladimir Putin has once again snubbed President Obama.

TODD: Russia has more than half the world's stockpile of nuclear materials and its safeguards haven't always been airtight.

BIENIAWSKI: There was an example several years ago where an insider employee at one of the Russian facilities at Luch was slowly taking out small quantities of nuclear material from that site.

PETER BERGEN, CNN NATIONAL SECURITY ANALYST: Certainly historically, I mean, I've been to Russia and gone to the summit at nuclear facilities. And you know, in the years after 9/11, it was very amateur the way they were holding these radioactive materials.

TODD: But experts say the Russians have since gotten better at securing nuclear material. Security analysts say if Putin had shown up at the summit, he could have shared intelligence on how to keep ISIS away from nuclear and radiological material. Either way the terror ground would have a tough time getting its hands on a nuclear weapon. But radiological material for a dirty bomb, often scored in hospitals and industrial complexes is far less secure. ISIS can access that. And ISIS supporters in the U.S. haven't been bashful about where they want to strike.

(On camera): This kind of target is really their aspiration, right?

BERGEN: Yes. So mass casualty attack. Clearly their aspiration. Radiological bomb if it went off here. Many people dead in the immediate vicinity. But the much bigger deal, Brian, is that it would disperse radioactive material all around downtown Washington, several block area. It would close down the city for many years.

(END VIDEOTAPE)

TODD: Now responding to the criticism of Putin for not showing up at the summit, a Russian official told us they're not sending a bad message and there's much more to nuclear security than just this summit in Washington -- Wolf.

BLITZER: Brian, they've given their own grievances for why Putin decided not to show up here in Washington, right?

TODD: That's right. They ticked off several of those reasons, Wolf. They say important countries with nuclear material like Iran are not participating. They say most of the key goals of the nuclear security have already been reached so why show up.

[17:00:04] And they said the U.S. is unfairly pushing its agenda on international groups like the U.N.'s nuclear watchdog and Interpol. So they've got a lot of grievances, Wolf, but a lot of this is personal between Vladimir Putin and Barack Obama.

BLITZER: All right, Brian Todd, reporting for us, thank you.

And stay with us. We're standing by for President Obama's live news conference on the nuclear summit. That coverage coming up.

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BLITZER: Happening now, fallout. Donald Trump scrambling to recover from what many are calling the worst week of his campaign. He's fighting falling poll numbers. His campaign manager is facing a misdemeanor battery charge and there's bipartisan anger surrounding his remarks on abortion. Can the GOP frontrunner regain his footing?